1. Field of the Invention
The present invention relates to formation of an image using a recording device having a plurality of light emitting devices.
2. Description of the Related Art
An image forming apparatus of an electrophotography system forms a latent image according to an image signal by scanning and exposing the surface of a photosensitive drum or belt with a laser beam or the like, and develops and transfers the latent image onto a print sheet, thus forming an image on it. The image forming apparatus of the electrophotography system is required to make concurrent scans using a plurality of light beams so as to increase the speed and resolution.
It is difficult to integrate a semiconductor laser diode (LD) of edge emitting type generally used as a light source of an optical scanning device, and the number of beams that can be concurrently used in scanning and exposure is as small as four. Hence, a vertical cavity surface emitting laser diode (VCSEL) including a two-dimensional array of a plurality of light emitting points is used as a light source of an optical scanning device. The VCSEL can be easily arrayed: when the VCSEL is used as a light source, a photosensitive member can concurrently undergo scanning and exposure using a larger number of light beams.
When a multi-beam array like the VCSEL is used, a heterogeneity of density in a lateral streak pattern (streak heterogeneity) is often generated in an output image due to a nonuniform exposure amount on a photosensitive member. In order to solve such problem, a method of suppressing a streak heterogeneity of a superimposition part one scan before by adjusting the laser light amounts at the upper and lower ends of the multi-beam array, and a method of setting uniform laser light amounts in the multi-beam array are available.
When the multi-beam array is used, a scan line formed by a laser beam that does not pass through an optical axis becomes a curve having an arcuate distortion (to be referred to as a curvature hereinafter) due to a distortion aberration of an optical system, and the curvature results in an image quality drop such as a heterogeneity of density of an output image. Conventionally, since the exposure amounts in the main scan direction (the scanning direction of a laser beam) are the same, the image quality drop due to the curvature cannot be decreased.